PROJECT SUMMARY Studying Natural SIV Reservoirs and Human Zoonotic Risk, we discovered in the last funding period that certain strains of SIV infecting Old World monkeys and African apes share unexpected antigenic cross- reactivity with HIV-1 in the functionally important V1V2 region of the envelope glycoprotein (Env) trimer apex (44). This antigenic conservation renders these viruses uniquely sensitive to HIV-1 elicited V2 apex broadly neutralizing antibodies (bNabs) and suggests that SIV Envs, because of their extensive backbone diversity but highly conserved trimer apex antigenicity, could serve to ?immunofocus? B cell responses in humans to this bNab epitope. Preliminary data in knock-in mice expressing the inferred unmutated common ancestor (UCA) of the V2 apex bNab CH01 (heavy chain only) provide supporting evidence: Immunization of these mice with a germline targeting SIVcpz Env SOSIP trimer (MT145.Q171K) elicited antibodies that not only neutralized viruses containing the autologous SIVcpz Env, but also viruses bearing Envs of heterologous HIV-1 strains (59). These results suggest that highly divergent SIVcpz (and other SIV) Envs can prime human V2 apex UCA B cells and induce bNab responses that cross-react with heterologous tier 2 HIV-1 strains (59). Utilizing our extensive knowledge of the evolutionary history, immunobiology and pathogenicity of the primate precursors of HIV-1 (summarized in 102 Progress Publications), we propose in this competing renewal of our MERIT award (R37 AI050529) to systematically explore the potential utility of SIVcpz and other SIV Envs as novel components of an AIDS vaccine. Our scientific premise is that the quaternary structure of the Env trimer apex is highly conserved across divergent SIV lineages because it mediates essential functions related to the biology of the unliganded Env glycoprotein, and that this structural conservation is reflected in conserved antigenicity. Our hypothesis is that evolutionarily divergent SIVcpz (and other SIV) Envs, when expressed as chimeric simian-human immunodeficiency viruses (SHIVs), mRNA-encoded cell surface expressed gp160 trimers, and/or virus-like particles (VLPs) in rhesus macaques (RMs), will elicit V2 apex bNabs that cross-react with HIV-1. To test this hypothesis, we will (i) characterize the extent of the V2 apex antigenic conservation across Envs from diverse primate lentiviral lineages and elucidate associated structure-function-antigenicity relationships relevant to vaccine design (Aim #1), (ii) define the potential of primate lentiviral Envs to elicit V2 apex bNabs in the context of SHIV infections (Aim #2), and (iii) determine if primate lentiviral Envs when delivered as mRNA/LNP or VLP immunogens can immunofocus V2 apex bNab responses in RMs, alone or in combination with heterologous SHIVs (Aim #3). Our application is timely because the SIVcpz MT145.Q171K Env SOSIP trimer has been selected by the NIH for GMP production and advancement to Phase 1 clinical testing in humans. The work we propose will provide an important complement and ?guidepost? to the immunogenicity studies planned in humans, and thus inform and accelerate AIDS vaccine development.